One of the things which pushes folks to convert to a newer motor is the cost of a rebuild and the modest performance inherent in the old B motor. While the various conversions sure bring plenty of power, they mostly bring plenty of cost. For a lot of B owners, the cost of a typical conversion is prohibitive, and the reward isn't valued enough to do. Some of the conversions are a lot of work and some need a shoe horn or scoop or cutting torch to make happen, and a lot of guys look at a scoop and similar mods as butcher work.
I've done a few conversions, and the 60 degree V6 is almost worth doing in the right situation. These fit in pretty easily and the resulting setup works quite well. Nonetheless, even with a good donor car and making some of the parts, a conversion burns through $2000 and a lot of DIY time at the very least. An expenditure of $4000 or so is more common. These rwd motors aren't so new anymore, either.
I've been pondering what might be another alternative which is a newer, more compact motor with 150 or more HP in RWD format for our cars. GM had put out a nice 4 banger 2.2 and 2.4 which looks to have plenty of pop for use in a B, and it looks quite compact. Probably drop in pretty readily. Anyone doing this one?

I was watching a friend pull apart his Mercruiser stern drive engine. After the water jacket and exhaust crap was out of the way, there sat a tidy little 4 cylinder. A few quick measurements and it seems like it would fit an MGB engine bay. The engine is basically half of a 460 Ford engine. New 460 pistons can be used. The head is iron but the block is aluminum. I asked an old hot-rod buddy about this engine and he said they used to put them in street rods hooked up to GM transmissions. The mount bolts/holes were already there. He said to stay away from the GM 4 cylinder versions of this in-line sterndrive engine and to use the Ford versions. GM parts were/are harder to get.
You can find these in marinas everywhere. Use caution if the engines were in saltwater service. You are safer if they were used only in freshwater areas.

Interesting as Scott's numbers are pretty close to what I will have spent for my 3.1. I have seen that BMC is now advertising a less expensive conversion using a GM 4 cylinder, so there must be interest. BMC's states their conversion uses the B exhaust system, rad etc and of course the proven T5 bolts right up. No tach conversion either.

Scott you mentioned in his first post an option with at least 150 horsepower. I think that the reason we do not see more 4 cylinder conversions is that in carborated form they rarely make 150 hp. The 3.7 liter Dave mentioned is an exception and a honkin big 4 banger to boot. On the other hand fuel injected four cylinders can easily make 150 hp or more. But if I look at conversions that have been done in the past I see more that are carburated that fuel injected. I wonder if it is that most converters are not that comfortable with wiring fuel injection that the 4 cylinder conversions are not that common.

Then again it could be that someone thinks: Well if I am going to go through the work of a conversion I might as well go to a larger motor anyhow.

New, modern, compact may be a little at odds with cheap. Seen a few posts about S2000 conversions and with bigger motors, LS lumps for newer options. Just the ECU for the S2000 is over a grand. Not sure what it would take to put together an electric one that charges off a hydrogen fuel cell (if you really want to think modern).

4 cylinder with posibilities of 200 hp and up, CCE is working on an engine kit for such engine, it takes time and testing to properly design such kit. There are other projects ahead for the MGB that I have in the works and need closure before I finish the engine kit.
The idea is to have a choice, 4 cylinder, 6 or 8 which I am also working on a simple V8 mount kit, no cross member, just mounts.

I've got a 1.8 Miata engine and gearbox in my 1973 MGB/GT. I wrote a three-part series of stories about it for Grassroots Motorsports magazine about 18 months ago. There are some buildup pictures on my website, too. It's a nice swap.

Aren't Honda engine reverse rotation from everyone else? I love Honda motors and have had a number, they are quiet, quick and easily modified. As for me I get itchy scratchy around stuff like ECMs so I am staying with carb on my 3.1 although I have kept the complete MFI intake system in case.

Just out of curiosity, Carl, does the BGT hood close on the Miata engine?

Doing a 2.3 ford turbo. Stock Hp is 195. I started with a turbo motor. But this can be done with a 2.3 Mustang Ranger motor, Garrett turbo, standalone ignition system and some forged pistons. Some modification of stock head will be necessary to prevent detonation. But some in Turboford are pushing 500+ and more. But not me.
http://s177.photobucket.com/albums/w224/mgman75/mgb%20project/?start=60

I'm installing a 2.0 liter 4 cylinder NISSAN motor (SR20DET). In theory, this should be great: approximately the same weight distribution and 250HP. This is not for the weak of heart however. Lots of custom fabrication, and I still haven't worked all the bugs out. And, I'm at three years and counting- I hope to have all the modifications and metal work done and ready for the paint shop this coming winter (said that last winter as well). Here is my progress:
http://www.mgexperience.net/member/Wilbur

We thought about one of the new engines GM has in the cobalt, the eco-tech but since they were FWD it made the change a bit tougher. So now I am watching for a classic mini that would like to try the eco-tech type engine.